RHD Boost – Enhancing RHDV effectiveness

Introduction

Rabbit haemorrhagic disease virus (RHDV) is a lagovirus in the family Caliciviridae that causes a mostly fatal infective disease in European rabbits. RHDV was first described in domestic rabbits in China in 19841. Until the late 1990s, there was only one known RHDV genotype however a new strain called RHDVa2 and other non-pathogenic or benign caliciviruses (bCVs) have been discovered3,4.

RHDV (Czech 351 strain) was first brought to Australia in 1991 to be tested as a potential biological control for introduced wild rabbits. This followed on from the successful release of myxoma virus in 1950, which killed an estimated 90% of wild rabbits in the first few years. Rabbits have developed genetic resistance to myxoma virus and its effectiveness has been reduced, but it still causes myxomatosis disease outbreaks in most rabbit populations and is an important component of rabbit control.

“There is a real need for Australia to start looking for the next generation of biological control agents”

In 1995, quarantined trials of the Czech 351 strain of RHDV began on Wardang Island, South Australia3. The virus escaped from the island, most likely aided by flies, and quickly spread across mainland Australia. RHDV drastically reduced rabbit numbers in many areas, particularly the arid inland regions5. However the virus did not have the same effect in cooler, higher rainfall parts and coastal areas of south-eastern Australia6.

Genetic resistance

Although rabbit numbers remained low after the establishment of RHDV, numbers have been on the increase since 2003. Researchers wanted to test if this was because rabbits were developing genetic resistance to RHDV7. Rabbits from nine sites across south-eastern Australia were tested for the development of genetic resistance. Rabbits were also collected from sites in central Australia and Queensland to determine whether climate influences the development of resistance in wild rabbits.

Results showed that rabbits in some regions now show relatively high levels of resistance to infection with the initially released Czech strain RHDV compared with other wild populations and domestic rabbits. This may underlie the recent rise in rabbit populations, particularly in inland Victoria and South Australia8. However widespread RHDV outbreaks are still seen in these areas, suggesting that the virus may also be changing to match growing rabbit resistance.

A new lagovirus

Scientists believe that bCVs have been present in Australia for a long time, certainly well before the Czech strain spread in 1995, but no-one had succeeded in isolating one. Varying levels of antibodies to bCVs were found to occur in rabbits at sites across southeastern Australia with generally high levels of the antibodies in rabbits from high rainfall areas and low levels of antibodies in rabbits from dry areas6.

Recently, the presence of a bCV (called Rabbit Calicivirus Australia 1 or RCV-A1) was isolated from wild rabbits3. It is likely that RCV-A1 came to Australia with the first rabbits around 150 years ago. Tests show that infection by RCV-A1 reduces the likelihood of death from the Czech 351 strain.

This may explain why RHDV appears less effective as a biocontrol agent in higher rainfall areas6. Scientists are currently investigating the geographic distribution and genetic diversity of this newly discovered strain, and are developing antibody tests that will help identify its presence in the field.

By working out how rabbits are developing resistance to RHDV and how RCV-A1 protects against RHDV, scientists hope to determine whether the introduction of new RHDV strains could be used to extend the effectiveness of the disease in overcoming Australia’s rabbit problem.

“These important findings will have a major influence on the way biological control of rabbits is used in Australia in the future”

RHD Boost

A new project called RHD-Boost is underway to evaluate alternative RHDV strains and how they could be safely and effectively released in Australia. Scientists will select candidate RHDV strains from overseas and test them to determine whether they can overcome the immunity to RCV-A1 and genetic resistance found in some wild rabbits. The new strains will also have to be proven to work in temperate and semi-arid parts of Australia.RHD-Boost

Further work

For now, researchers continue to study rabbits and the virus, RHDV, in an effort to enhance its effectiveness and future use as a biological control agent. However, studies are underway to see if there are any more potential biocontrol agents that might be used in the future. Biosecurity SA and the Invasive Animals Cooperative Research Centre (IA CRC) are searching for a new disease, bacterium, virus or biological agent with potential as a control agent for rabbits. This long-term rabbit “bio-prospecting” project will investigate any organism from any part of the world, which might be effective against rabbits, humane and safe for humans and other animals. However the release of any new control agent must also be accompanied by greater landholder education and increased community vigilance. The key to successful, long-term rabbit management is a coordinated approach – ongoing control over a broad area, integrating best practice conventional control techniques including poisoning, ripping and fumigation with biological control to maximise the impact on rabbits9.